The present invention is a device capable of measuring electrostatic potentials over the range of about 0 to -1000 volts with an absolute error of less than 5 volts and with insignificant current flow into the input of the device. Generally, such devices include a probe or sensor assembly working in conjunction with an associated voltmeter assembly which receives the signals from the probe and produces an output signal. Subsequently, the output signal may be used to drive an indicator, or to control an electrostatic process as a function of the measured electrostatic potential. The outputs signal must represent the maximum of minimum voltage measured and be responsive so as to produce a full scale change in less than 10 milliseconds. Thus, the features of the present invention may be used in an electroreprographic system to control a xerographic process. These electrostatic voltmeters, or ESVs, are particularly well suited for measuring photoreceptor surface charge, which in turn allows for the automated adjustment of machine characteristics to achieve high quality reprographic output.
Heretofore, it has been recognized that, in a non-contacting ESV, the sensing probe or electrode should be modulated with respect to the field being measured in order to accurately measure the field. Moreover, two methods of achieving the required modulation of the electrode are known, the first requiring a stationary electrode and a vibrating element, or vane, to modulate the field which reaches the electrode, as described by U.S. Pat. No. 3,921,087 to Vosteen (issued Nov. 18, 1975), or by U.S. Pat. No. 4,149,119 to Buchheit (issued Apr. 10, 1979). The second approach utilizes a moving electrode, affixed to the end of a vibrating element as disclosed in U.S. Pat. No. 3,852,667 to Williams et al. (issued Dec. 3, 1974), or alternatively as disclosed in U.S. Pat. No. 5,212,451 to Werner (issued May 18, 1993).
Moreover, numerous approaches are recognized to process the signal produced by the sensing electrode, thereby enabling the measurement of the electrostatic field potential. Some signal processing approaches are illustrated in the following disclosures which may be relevant:
U.S. Pat. No. 3,644,828 to Gathman (issued Feb. 22, 1972) teaches a direct-current high-voltage electrometer for measuring potentials, where the a high-input impedance is provided by a guard voltage supplied by a charged capacitor.
U.S. Pat. No. 3,667,036 to Seachman (issued May 30, 1972) discloses electrometer amplifier circuits for measuring the potential of the electrostatic charge formed on an insulating surface. The circuit includes a probe assembly consisting of probe and guard electrodes. The output of the probe electrode is connected to a high impedance circuit which comprises a Metal Oxide Field Effect Transistor (MOS FET) in a source-follower configuration.
U.S. Pat. No. 4,027,240 to Meade (issued May 31, 1977) discloses a voltmeter used in detecting an electronic voltage signal in an ordinance firing circuit, where the voltmeter is protected by a pair of back-to-back parallel-connected limiting diodes. A first limiter is used to reduce the amplitude of the detected signal, while the second limiter prevents overloading of an indicator.
U.S. Pat. No. 4,061,983 to Suzuki (issued Dec. 6, 1977) discloses a transistor amplifier including a bipolar transistor supplied with an input signal and a field effect transistor (FET) which is directly connected to an output electrode of the bipolar transistor to amplify the applied signal. A protective circuit senses the load impedance and activates a voltage signal which ultimately results in the lowering of the gate potential on the FET to prevent the FET from damage.
U.S. Pat. No. 4,063,154 to Andrus et al. (issued Dec. 13, 1977) discloses a feedback circuit for the probe of a direct-current electrostatic voltmeter to reduce the spacing sensitivity thereof.
U.S. Pat. No. 4,149,119 to Buchheit (issued Apr. 10, 1979) teaches an electrostatic voltmeter or electrometer which includes a probe sensor for sensing electrostatic charge present on a test surface. The probe sensor is modulated using a rotating vane or shutter arrangement. The probe is also conditioned to receive both A.C. and D.C. signals which are amplified by a D.C. amplifier, where the A.C. signal from the probe is fed back to the D.C. amplifier to stabilize its output.
U.S. Pat. No. 4,330,749 to Eda et al. (issued May 18, 1982) teaches an electrometer apparatus for measuring the electrostatic potential on the surface of a photoconductive drum. The apparatus consists of an electrode which is placed near the surface on which the electrostatic potential is to be measured. A potential proportional to the surface potential is induced in the electrode and applied to the input of an amplifier with high input impedance. The amplifier has a MOS FET input stage with a high input impedance and a low bias current.
U.S. Pat. No. 4,673,885 to Lewiner et al. (issued Jun. 16, 1987) teaches a device for reading the quantity of electrical charge borne by a dielectric sheet, where a probe is used to scan the surface of the sheet. The probe is in turn connected to a circuit adapted to measure the charge induced on the probe.
U.S. Pat. No. 4,853,639 to Vosteen et al. (issued Aug. 1, 1989) discloses a non-contacting type electrometer apparatus for monitoring the electric potential of a test surface. A sensing integrator is used in conjunction with a pre-amp and a high-gain operational amplifier (opamp) to provide an improved high-frequency response.
U.S. Pat. No. 4,797,620 to Williams (issued Jan. 10, 1989) discloses a non-contacting electrostatic detector which eliminates the use of high-voltage circuitry in non-space dependent, high-voltage electrostatic monitoring devices. An A.C. voltage, having the same frequency as the modulator frequency, is used to produce a zero net current flow, so that the magnitude and phase of the output signal are proportional to the magnitude and polarity of the electrostatic potential being monitored.
U.S. Pat. No. 4,804,922 to Sometani et al. (issued Feb. 14, 1989) and U.S. Pat. No. 4,868,907 to Folkins (issued Sep. 19, 1989) both disclose devices which proportionally convert electrostatic voltage into current.
U.S. Pat. No. 4,878,017 to Williams (issued Oct. 31, 1989) teaches a non-contacting electrostatic voltage follower having a response speed independent of the frequency of modulation of the capacitance or electrostatic field between a detector electrode and the measured surface. The voltage follower is capable of following both static and dynamic characteristics of an external field or potential to be measured.
U.S. Pat. No. 4,973,910 to Wilson (issued Nov. 27, 1990) teaches an electrostatic analyzer that incorporates a field effect transistor (FET) as a sensor used to convert electrostatic voltage into a proportional current. The sensor is described as a semiconductive device having n-p-n junctions. In operation, a zero-field reference is used to alter the base potential of the sensor, thus forming a zero-field condition. The surface potential difference is then determined as a function of the sensor base voltage, which is directly measured by a voltmeter.
U.S. Pat. No. 5,065,102 to Takanashi et al. (issued Nov. 12, 1991) teaches an apparatus for detecting the distribution of electric surface potential in a device employing a low voltage field effect transistor in the sensing device. Inaccuracy resulting from the field effect transistor are overcome by subjecting the gate of the transistor to an electrostatically induced voltage corresponding to the surface potential being measured, and periodically shunting the gate of the transistor to ground in response to a reset pulse.
FR-A-2 450 461 by Mettier (published Sep. 26, 1980) illustrates a circuit for the detection of electrostatic charge.
The Kumada publication (JA-62-90564), published Apr. 25, 1987, illustrates a circuit for measuring surface potential, including an independent power source which provides power to an impedance converter circuit which is separated from a signal processing circuit.
EP-A-0 274 995 by Chieli (published Jul. 20, 1988) teaches a circuit for the linear measurement of current through a load, where a pair of field effect transistors are interconnected to form a current mirror.
In operation, the electrostatic voltmeter described in U.S. Pat. No. 5,323,115 and in U.S. Pat. No. 5,270,660 to Werner, Jr. et al. issued Dec. 14, 1993, both hereby incorporated be reference, attempts to "follow" a changing input signal where the finite transconductance of a field-effect transistor (FET) employed in an electrostatic voltmeter produces a variation in the current passing through the FET in response to a voltage variation across the FET. This variation can also produce a transient direct-current signal into a coupling capacitor used in the circuitry to process the input signal. The gate-to-source resistance of the high-voltage MOS FET is large in order to generate as large a gate-source modulation voltage as possible. The drain-to-gate capacitance feeds into the "resistor" and while the capacitance is small (typically about 20 fF)the voltage changes generate sufficient current through the capacitance to generate a potential from gate-to-source capable of driving the FET out of the normal operating range. Lastly, as a result of imperfect modulation, the modulator is a capacitance coupling (between the ESV circuit and the photoreceptor surface being characterized) applied directly into the gate of the high-voltage MOS FET. Any transient DC changes on the surface being characterized are fed directly to the gate; this can also drive the FET out of the normal operating range. Although the more efficient the voltmeter modulation is the more less serious the transient problem becomes, there is still a processing burden on the circuits employed for AC/DC separation. Accordingly, the present invention is intended to resolve the current variation and transient direct-current signal problems noted above by shunting large current variations into the common substrate node found in an FET and, specifically, by employing a high-voltage differential cascode MOS FET pair as will be described in further detail.
In accordance with the present invention, there is provided an apparatus for generating a low voltage signal proportional to an electrostatic potential on a surface, comprising:
a non-contacting sensor for producing an input signal representative of the electrostatic potential on the surface;
a high-voltage source adapted to produce a first potential of first polarity;
a high-voltage level shifting circuit, powered by said high-voltage source and including an input conditioning circuit for reducing transient variations in the input signal produced by said sensor, said high-voltage level shifting means generating a first signal referenced to ground in response to the conditioned input signal output from the input signal conditioning circuit; and
a circuit, connected to receive the first signal from said high-voltage level shifting means, for converting the first signal to a low voltage signal, said low voltage signal being referenced to ground potential.